Synthesis of AlxCoCrFeNi HEA thin films by high power impulse magnetron sputtering: Effect of substrate bias voltage

In this study, AlxCoCrFeNi high entropy alloy (HEA) thin films were deposited by high power impulse magnetron sputtering (HiPIMS) to explore the effect of substrate bias voltage on their microstructure, mechanical properties, and corrosion and tribocorrosion behavior. Si (100) and AISI 304 stainless steel (SS) were used as substrates, with the latter also serving as a reference for comparative purposes. Higher bias voltages promoted the formation of a dense, fine-grained microstructure, which positively affected the electrochemical performance of the films in a 3.5 wt.% NaCl aqueous solution. When the bias voltage exceeded -70 V, the Al content decreased from 10 to 5 at.%. As a result, the crystal structure transitioned from a dual-phase face-centered cubic (FCC) + body-centered cubic (BCC) structure to a single-phase FCC structure. While the residual stress state within the films shifted from tensile to compressive with increasing bias voltage, their hardness remained relatively constant at similar to 9 GPa. However, some differences were found in terms of tribocorrosion behavior, with the wear rate exhibiting a non-linear trend with increasing bias voltage. Optimal wear performance was achieved at intermediate bias voltages, while excessive bias voltage values resulted in increased material loss. Nevertheless, all films demonstrated superior corrosion and tribocorrosion resistance compared to the AISI 304 SS substrate. These findings underscore the importance of adjusting the substrate bias voltage in the deposition of HEA thin films via HiPIMS to optimize their performance in wear- and corrosion-prone applications.